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CRISPR-Cas systems are bacterial immune systems that target invading nucleic acid. The hallmark of CRISPR-Cas systems is the CRISPR array, a genetic locus that includes short sequences known as “spacers”, that are derived from invading nucleic acid. Upon exposure to an invading nucleic acid molecule, bacteria/archaea with functional CRISPR-Cas systems can add new spacers to their CRISPR arrays in a process known as “adaptation”. In type I CRISPR-Cas systems, which represent the majority of CRISPR-Cas systems found in nature, adaptation can occur by two mechanisms: naïve and primed. Here, we show that, for the archetypal type I-E CRISPR-Cas system from Escherichia coli, primed adaptation occurs at least 1,000 times more efficiently than naïve adaptation. By initiating primed adaptation on the E. coli chromosome, we show that spacers can be acquired across distances of >100 kb from the initially targeted site, and we identify multiple factors that influence the efficiency with which sequences are acquired as new spacers. Thus, our data provide insight into the mechanism of primed adaptation.[This paper has been peer reviewed, with Ailong Ke (Cornell University) serving as the editor. Reviews and point-by-point response, and a marked-up version of the edited manuscript are provided as supplementary files.] |
